The present state of the art in stair building does not differ much from the earliest known stairs, except for the addition of different categories of stairs, such as spiral stairs. Going forward from time to time stairs have been curved, angled, and stacked. Risers have been closed, open, and cantilevered, and stringers have been single, double, cantilevered and various forms thereof. Stairs have been dramatic and plain, and they have been made of wood, stone, metal and all types of combinations of these and other materials. However all of the above known types of stairs fit one of the following categories of conventional stairs: single straight run, double run, multi-run, curved and spiral.
Single Straight Run Stairs
Single straight run stairs, which are the most basic type of conventional stairs, are the easiest to climb, and allow passing of up and down traffic at the same time. Such stairs provide the shortest access to an elevated area. In modern times, risers are usually 6.5 to 7.5 inches high and treads are usually 9.5 to 12 inches deep. For years designers have used 70 to 72 inches as a multiplier for the preferred tread and riser. That is, a stair with a 6 inch rise should have a preferred tread of near 12 inches, which is 6×12, or 72. As another example, a 4 inch rise (such as used on a sidewalk area) might have an 18 inch tread (4×18 which again equals 72). About 72 inches has been found to be a good stride for adults. However, when the stairs are mainly intended for children, the 72 value can be reduced.
Straight run stairs are usually supported at both the upper and lower ends. At the lower end, most often the first riser and stringer lower end sits on the floor or a prepared support, and at the upper end, the stringers rest against or are fastened to the opening header (much like the top of a ladder leaning against a structure). Straight stairs usually have an open area above them, and most have good clearance so carrying boxes or furniture can be easy on a straight run stairway. Some stairs that rise 8 to 10 feet have a single straight run of 12 to 18 risers. Single straight run stairs are a very efficient design for stairs. They take up the least square feet of floor space of the various conventional forms of standard straight stairs, and with the single straight run design, a user walks the least distance between floors. However, the design does not work at all locations, and there can be several issues in the single straight run design. For example, this design requires one tread space for each riser, so even a small stair that is 36 inches wide with 14 risers and 10 inch treads will need a floor space of about 36 inches×140 inches on both floors. Such a large amount of floor space between floor framing members is something that is hard to find in some structures. To create a 140 inch long opening, heavy framing is needed at the stair header and parallel to the stair. Additionally, with regard to safety, having between 12 and 18 risers in a row can increase the fall hazard, and the distance one will tumble if there is a fall is also significant.
Double-Run and Multi-Run Straight Stairs
Straight stairs can also be made to have two or more runs (i.e., called double-run stairs and multi-run stairs, respectively). While such double-run and multi-run stairs have the same traffic flow attributes as single straight run stairs, they improve on the fall issue because they can shorten the in-line riser run distance because the straight run is broken up by one or more platforms. Further, such stair designs cut the front to back depth of the floor opening down somewhat. For example, a fourteen riser stair 3 feet 0 inches wide would need 70 inches for 7 treads and 36 inches for platform depth or about 106 inches total verses 140 inches for the straight run. However, the floor space and the hole at upper floor would now be about 72 inches×106 inches on a 14 riser 36 inch wide stair when you add in the 36 inch×72 inch platform and the return run up from the platform. Double-run straight stairs also add walking distance to the straight run stair design, as distance and extra steps are needed to make the turn at the platform, because steps taken on the platform do not make any vertical progress toward the upper floor. Multi-run stairs increase the walking distance further due to the inclusion of multiple platforms. As with the standard straight run stairs, double-run and multi-run stairs usually have an open area above them, a lack of tight corners or curves, and most also have good clearance to allow the easy movement of boxes or furniture.
Additionally, due to the inclusion of the platforms on double-run and multi-run stairs, platform support is needed. This support could be in the form of walls, hangers, struts or some combination thereof. The platform becomes an intermediate floor that is used to support the upper end of the lower stringer and the lower end of the upper stringer. On some stairs, there can be several platforms between floors, all of which act as intermediate floors and all of which must be supported with hangers, struts or walls.
Spiral Stairs
Spiral stairs are often used when floor space is limited. For example, a spiral staircase with a 36 inch wide tread (72 inch diameter), only needs a 72 inch×72 inch floor space and an upper floor opening of similar size. However a 36 inch wide spiral stair tread should not be considered as being equivalent to a 36 inch tread on a single or double run straight stair. This is the case because the spiral tread is wedge-shaped, while straight stairs generally have rectangular treads. On the wedge-shaped spiral stair tread, the ideal walking line is about 16 inches from the center of the stair tread. At that location, the tread and riser are similar to the other types of stairs. When a person walks that line, risers are in the 6.5 inch to 7.5 inch range, and treads are in the 10 to 12 inch range, which is very similar to the standard stairs. However if a person walks a spiral stair on the outside near the 72 inch diameter, the risers are the same (6.5 to 7.5 inches), but the tread grows to the 14-17 inch range. This larger stride is well out of the comfort range for most adults, as it a very long stride. On the inside (i.e., near the center of the tread), the stair risers are again in the 6.5 to 7.5 inch range, but here the tread is very small, for example possibly only in the 5 to 7 inch range. Such a small range for the tread, is difficult to walk going up, and even more difficult to walk going down, as well as being dangerous.
Additionally, spiral stairs are generally very tight, so it is very difficult to carry boxes or furniture up or down a spiral staircase. The stair tends to wrap over itself, so for the first six or seven risers of a one story stair there is less than 48 inches between the hand rail and treads above. Moving anything large up or down a normal spiral staircase is a major challenge due to limited space and the tight radius. Additionally, due to the tread shape and the limited space on most spiral staircases, users walking in opposite directions will have a hard time passing each other. Thus, although spiral stairs are aesthetically pleasing and take up the least floor space of the different types of stairs, they are very impractical when high volume use is required or where product needs to be moved by stairs. Seldom will you find spiral stairs as the only stair between levels, the main exception being residential home use for access to lofts and basements where space floor space is very limited. From time to time spiral stair builders have added three or four straight run treads to the top or bottom of the standard spiral by attaching a straight stringer to the main support pole. Such a system does not include a bent pole stringer, but instead includes a main pole that extends from a starting floor to an upper floor with a secondary straight stringer attached at an angle thereto and extending to either the starting floor or the upper floor.